It is known that an attenuator literally refers to a device/circuit used for attenuating power. For example, a power can be attenuated as desired by employing diverse consumptive elements such as resistors. There has been frequently used a technique that divides a power through a resistor network or an array of resistors and makes a portion of the distributed power terminated.
Generally, such attenuators are classified into a fixed attenuator having a fixed attenuation amount and a variable attenuator having an arbitrarily adjustable attenuation amount.
The fixed attenuator for intentional attenuation should be designed to maximally suppress issuance of noise above a certain amount and also not to incur a phase difference since it is used only for the purpose of lowering power level in diverse cases.
This fixed attenuator is used in various ways. In many cases, the fixed attenuator is utilized for fitting power levels of plural stages to a required level; and especially used to measure an output of a high output device such as a power amplifier. If a high output of tens of dBm or more is directly input to a measuring device, such device may be damaged. Therefore, it is possible to safely use the measuring device if an output value is measured with an attenuator having an accurately known amount of attenuation prepared at an output stage and then compensated in a numerical expression.
On the other hand, the variable attenuator is employed for a power control, a gain control, temperature compensation and so on because a user can arbitrarily change magnitude of Radio Frequency (RF)/microwave signal in transceivers of various wireless communication systems such as a personal communication system, a military communication system, a satellite communication system, etc.
A high frequency variable attenuator can be implemented in a type of an analog or digital attenuator.
An amount of attenuation of the analog attenuator is varied nonlinearly with respect to an amount of a control voltage and current; and therefore, there is a problem that a complicated control circuit is needed to control the amount of attenuation thereof accurately. Another problem is that the amount of attenuation is largely varied due to sensitivity to the temperature and manufacturing process.
Meanwhile, the digital attenuator has high linearity and is strong to high power, even under conditions of a wide operating temperature range and a large variation of manufacturing process of Monolithic Microwave Integrated Circuit (MMIC), compared to the analog attenuator. Also, the digital attenuator is preferred to the analog attenuator owing to easiness of control of the attenuation amount and its accuracy.
This digital attenuator is configured in units of 1 dB, 2 dB, 4 dB and 8 dB, etc., connected in series, each of which forms an individual bit and is switched to obtain a desired amount of attenuation. For the switching of the individual bit, a switching device is used. As such switching devices, there exist a Field Effect Transistor (FET) and a High Electron Mobility Transistor (HEMT), wherein a resistance between a drain terminal and a source terminal is controlled in a range from several ohms to several Kilo ohms by a voltage applied to a gate terminal.
The serial bits provided by those switching devices are weighted in a binary way starting from the Least Significant Bit (LSB), which is minimum resoluble level. The Most Significant Bit (MSB) is determined by the maximum attenuation amount. The minimum attenuation amount is obtained when each of the individual bits is on, while the maximum attenuation amount is obtained when each of individual bits is off. The minimum attenuation amount is referred to as a zero or reference state insertion loss of the attenuator.
As important performance items of the digital attenuator, there are accuracy of attenuation, an insertion loss, impedance matching at input and output ports, a power handling, a phase change, a size of the attenuator, facilitation of implementation of control circuit, etc.
Details of the important performance items of the digital attenuator are as follows. First, the accuracy of the attenuation of the digital attenuator implies degrees of error for each individual bit in a fixed attenuation amount within an operating frequency range. Normally, the accuracy of the attenuation is considered to be good when it is within ½ of the LSB.
The reference state insertion loss of the digital attenuator should be as low as possible. This is because the digital attenuator of low loss has many advantages in various aspects of power consumption, a circuit size, cost, and so on.
The input and output impedance matching of the digital attenuator should be very good so that the insertion loss and phase ripples are not issued within the operating frequency range when it is connected in series to a phase shifter or an amplifier. The input and output impedance matching of each individual attenuation bit affects overall performance of the attenuator. By mismatching of the individual bits, interference is taken place within an MMIC chip, thus deteriorating the amplitude and phase performance of the overall chip.
The power handling characteristic, as defined as Input 3rd order Intercept Point (IIP3) in the attenuator, means linearity of the attenuator. Accordingly, the IIP3 should be high in order to use the attenuator in high power level.
A portion of systems such as a phase array antenna require that an insertion phase of the attenuator is not changed ideally for all attenuation states if possible.
In addition, when the size of the attenuator is smaller, it is more advantageous since its weight and cost can be reduced. A control circuit that is able to be simply implemented is preferred in terms of facilitation of implementation thereof as another important item.
As described above, the every important performance items cannot be satisfied simultaneously; and therefore, compromise is needed among those performance items. In order to satisfy the performance items as discussed early, various types of digital attenuators are proposed in many documents. Among them, there are disclosed “Distributed T Digital Attenuator” in U.S. Pat. No. 5,309,048; and “Switched Pi Digital Attenuator” in U.S. Pat. No. 5,448,207. Another prior art is “Switched T Digital Attenuator”, as illustrated in FIG. 1. The operating frequency bands of these digital attenuators are all narrowband, which exist within 20 GHz, and accuracy of the attenuation is also lowered. A more detailed description thereon will be provided referring to FIG. 1.
FIG. 1 illustrates a structure of the conventional switched T digital attenuator.
As shown in FIG. 1, in the structure of the switched T digital attenuator, a first resistor 11, which is a resistive element for attenuating an input high frequency signal, is connected in series to an input port 16. A second resistor 12 is coupled in series between the first resistor 11 and an output port 17. And, a first switching device 14 is connected in parallel with the first and second resistors 11 and 12 between the input port 16 and the output port 17. A second switching device 15 and a third resistor 13 are connected in series between the first and second resistors 11 and 12, wherein the third resistor 13 is shorted to a ground.
In other words, in the switched T digital attenuator as shown in FIG. 1, if the first switching device 14 is shorted by a 1st voltage 18, which is a control voltage, and the second switching device 15 is opened by a voltage 1st voltage 19, the attenuator becomes a low loss state where the input signal is almost passed. On the other hand, if the first switching device 14 is opened and the second switching device 15 is shorted, the attenuator has a constant attenuation amount, which is mainly determined by the resistance values of the first, second and third resistors 11, 12 and 13. A difference value between the attenuation amount mainly determined by the resistance values of the first, second and third resistors 11, 12 and 13 and the above-described insertion loss in the low loss state indicates an attenuation amount varied by the 1st voltage 18 and the voltage 1st voltage 19, which are the control voltages. Here, the voltage 1st voltage 19 is the control voltage, which is indicative of an inverted voltage of the 1st voltage 18. Therefore, if the 1st voltage 18 is in a high level, the voltage 1st voltage 19 is in a low level, and vice versa.
Meanwhile, a T-type attenuator structure can be formed by the first, second and third resistors 11, 12 and 13 of resistive elements, which are remained after removing the switching devices 14 and 15 from the switched T digital attenuator as shown in FIG. 1. The following equation 1 represents resistance values of the resistive elements according to the attenuation amount in the T-type attenuator structure formed by the first, second and third resistors 11, 12 and 13 of the resistive elements.
                                          R            ⁢                                                  ⁢            1                    =                                                                                          10                                          A                      /                      10                                                        +                  1                                                                      10                                          A                      /                      10                                                        -                  1                                            ⁢                              Z                0                                      -                          R              ⁢                                                          ⁢              3                                      ,                              R            ⁢                                                  ⁢            1                    =                      R            ⁢                                                  ⁢            2                          ,                              R            ⁢                                                  ⁢            3                    =                                    2              ⁢                                                                    z                    0                    2                                    ⁢                                      10                                          A                      /                      10                                                                                                                          10                                  A                  /                  10                                            -              1                                                          Eq        .                                  ⁢                  (          1          )                    wherein A denotes an amount of attenuation (dB) and Zo indicates characteristic impedance of the input and output terminals of the attenuator.
The conventional switched T digital attenuator has problems in that an error between a preset attenuation amount and an actual attenuation amount is large, and the error therebetween becomes large as the frequency increases. The problems are occurred by capacitive coupling of intrinsic parasitic of the switching devices, which have frequency-dependent characteristic. Thus, these problems act as great limit factors in establishing a broadband of the digital switch and also in the accuracy of attenuation.
Consequently, in the conventional digital attenuators proposed in the above-described documents, the operating frequency bands are all narrowband, which are within 20 GHz, and the accuracy of the attenuation thereof is lowered.